Destaging threshold HUS VM / VSP / AMS2500

We use CLPR (Cache Partitions) for external systems for several reasons (slow external system may impact the overall system), as recommend by HDS.

But what about CLPR for internal volumes?

In general, at which threshold starts the "emergency destaging" process, if the cache is flooded (for external and internal volumes) and does this process may have an impact to the overall system (blocking host IOs?)?

Situation: We have to "separate" two hosts for testing purposes with a dedicated CLPR and Ports. But what happens, if these two hosts start flooding the CLPR?

In Backup mode the USP will retain all user data in cache and control data in shared memory and will not destage to disk. How long the batteries can maintain CM and SM depends on how much CM and SM you have installed as well as the condition of your batteries.

In destage mode, the contents of CM are destaged and then the contents of SM are also destaged. Yes the batteries spin the disks while this destaging takes place. Once data is destaged the subsystem shuts down and depending on some factors CM and SM may be backed up by battery, or they may not. If they are not then the start-up of the subsystem takes a little longer because the contents of CM and SM have to be staged from disk.

Also, destage mode is not always possible. E.g it is not really supported if you have external storage hung off the back of the USP, as when the power fails the USP will only destage to its own internal disks and not to external storage. Also it [i]may[/i] not be available if you have more than 2 ACP pairs installed. Best speaking to your CE to find out the latest supported configurations.

And finally, the USP will wait for 1 minute after detecting a power outage before it does anything. This is a good feature in case the power goes out and comes back almost immediately. No point in shutting down if it was just a blip. But once the destage begins, you cant stop it.